1. Field of the Invention
This invention relates broadly to the field of convenience closures for full-open type can ends. More specifically, this invention relates to an improved rivet formation for attaching a gripping tab to an end panel of such a can end.
2. Description of the Related Technology
One form of self-opening can which is in wide use is the so called full-open can, in which a peripheral score, generally circular in configuration, is formed in the end panel at or adjacent to the periphery thereof to permit its complete removal. Full-open type cans are to be distinguished from those self opening cans which have a comparatively small removable section which, when opened, provide a comparatively small hole for dispensing the product. The latter type of can end is only appropriate for packaging soda, beer, or other liquids. Full-open type cans, on the other hand, are suitable for packaging solid products such as candy, nuts, meats, or ground coffee.
A conventional full-open type can end of the type disclosed in U.S. Pat. No. 5,232,114 to Zysset is depicted in FIG. 1. Can end 10 conventionally includes an end panel 12 and an edge-curled peripheral flange 14 that is adapted to be interfolded with an end flange of a can body. At or adjacent to the periphery of end panel 12 is a peripheral score 16 which is indented into the end panel 12 and defines a fixed can end portion 18 and a removable portion or panel 20. A gripping tab 22 is mounted to the end panel 12 by means of a rivet formation 24. The gripping tab 22 is positioned close enough to the peripheral score 16 so that when its inner end is rocked upwardly to cause its outer end to move downwardly and exert a downward force on the end panel 12 at or near the peripheral score 16, a portion of the end panel 12 is bent downwardly to initiate rupture of the peripheral score 16. Thereafter, an upward and backward pull on the gripping tab 22 by the user induces tearing of the metal in the peripheral score 16 on both sides of the area of initial rupture to complete detachment from the can of the removable portion or panel 20 of the end panel 12.
It is also known within the prior art to employ a vent score behind the rivet formation 24 toward the center of the end panel 12 in order to relieve internal pressure or vacuum that exists within the container, and thereby avoid implosion or explosion as the container 12 is opened by rotation of the gripping tab 22 about the rivet formation 24. The most frequently used shape for the vent score is curved like a man's mustache and therefore these scores are generally known as “mustache scores.” For the aforesaid purposes and to promote flexibility and to provide a hinge point for the tab 22 when it is lifted, a mustache score 34 may be formed in the end panel 12, as is illustrated in FIG. 1. Mustache score 34 typically has an inner portion 36 that is defined on a raised area 26 of rivet formation 24 having an outer edge 27, and at least one outer portion 38 that is defined on the outer surface of end panel 12. Raised area 26 includes an oval shaped first operation bubble 40, which is elongated along an axis which is a chord perpendicular to a radius of end panel 12. Raised area 26 further includes a second operation bubble 30, which is defined centrally with respect to the first operation bubble 40. The inner portion 36 of mustache score 34 may be curved about the radial inner periphery of the second operation bubble 30. The outer portions 38 of the mustache score 34 would typically be shaped so as to flare outwardly from the respective inner portions 36, and would typically end up at their extreme outer ends to be substantially parallel or slightly curved away from the peripheral score 16.
Although conventional mustache scores such as the one that is described above are generally effective in preventing violent explosions or implosions during the opening of containers that are moderately pressurized or underpressurized, they tend to be ineffective in preventing explosions during the opening of highly pressurized containers. A highly pressurized container for purposes of this document is a container that is pressurized at least 25 pounds per square inch over ambient pressure conditions. A need exists for an improved easy open can end that can be used with highly pressurized containers and that is capable of effectively reducing the likelihood of a violent explosion or implosion occurring when such a highly pressurized container is opened.